This invention concerns improvements in or relating to infra-red optical systems and relates more particularly to afocal optical systems for use at infra-red wavelengths.
Afocal optical systems, i.e. systems having an infinite focal length and therefore no apparent focussing action, are well known per se and are used in several applications. The present invention relates to such a system which can be used, in particular, in circumstances where it is required that a beam of infra-red radiation from a distant source and incident on the optical system be reduced in diameter. Such reduction of beam diameter enables a comparatively small, high speed reflective scanning system to be employed in order to provide the requisite scanning of the field of view across an imaging lens plus detector array.
An optical scanning head of constant angular motion has to operate in a collimated radiation section in order to maintain focus across the field of view and minimise image distortion. An essential in order to prevent vignetting is that the exit pupil of the afocal system should be coincident with the operative facet of the scanning head, i.e. its in-action reflective facet. In order to give a good field of view in the real world and also to minimise the number of facets on the scanning head a wide field of view is required in the image space of the afocal system. The afocal optical system must, therefore, be capable of providing a wide image space field angle at an external exit pupil that is situated a convenient distance after the last optical element of the system.
Further, the space in which the afocal system is to be fitted is usually very limited, and this introduces a requirement for a compact length. Also, the image quality given by the afocal system should be close to the limit set by diffraction as this quality is not particularly high because of the long wavelength of the infra-red radiation.
It has previously been proposed in U.S. Pat. No. 4,199,217 to provide an "eye-piece" system for a non-Gallilean afocal infra-red optical system, otherwise referred to as an infra-red afocal telescope, which "eye-piece" system comprises a back lens element of positive power having a back surface which is convex or concave and a front surface which is convex, and a front lens element which is of meniscus form having a back surface which is convex and a front surface which is concave with the radius of curvature of the convex back surface greater than that of the concave front surface but the thickness of the front element being such as to give it positive power.
In that patent, and in the present description, the term "eye-piece" is used although the ultimate image is not viewed by the human eye but is, for example, scanned across an infra-red detector array, and the term is to be construed accordingly. Also in that patent, and in the present description, the front of the system is considered to face towards the scene or object from which infra-red radiation is received while the back of the system is considered to face towards the exit pupil. The terms front and back when used in particular in relation to elements of the system and to the surfaces of such elements are to be construed accordingly.